The present invention relates to industrial controllers used for the control of industrial processes and machines, and in particular to a rugged assembly method for compact industrial controllers.
Industrial controllers are special purpose computers used for controlling industrial processes and manufacturing equipment on a real-time basis. Under the direction of a stored program, a processor in the industrial controller examines a set of inputs reflecting the status of the controlled process and changes a set of outputs controlling the industrial process. The inputs and outputs may be binary, that is on or off, or analog, providing a value within a continuous range. Typically analog signals are converted to binary words for processing.
The inputs and outputs are processed by input/output (I/O) circuitry which performs any necessary data type conversion, level shifting, isolation and amplification of the signals to and from the processor so as to be compatible with the signals required by the industrial controller. Processor and I/O circuitry for industrial control are well known in the art.
Compact industrial controllers may be produced in which the processor and the necessary I/O circuits are in a single housing. It is desirable that the base of the housing, defining its footprint when attached to the back of a cabinet, be small to conserve cabinet space. This small footprint may be accommodated by dividing the internal circuitry of the controller into multiple circuit cards stacked one on top of another.
Interconnections between the circuit cards may be accommodated by conductive connectors typically having gold plated pins necessary to communicate the low voltage logic level signals used by the processor. The multiple cards must be rigidly mounted so that vibration to be expected in the industrial environment does not cause abrasion of the gold plating layer.
In addition to holding the processor and I/O circuits, such a compact industrial controller must support terminal blocks or the like providing a means to connect the signals of the controlled equipment to the I/O circuitry. Two sets of terminal blocks separated across the front of the housing may be provided, one for input signals and one for output signals. The terminal blocks may also provide connections for line power in, user power out, and other signals known in the art.
Normally the terminal blocks will be attached directly to the circuit card holding the I/O circuitry by mechanically rigid conductors being continuations of the stampings which form part of the connector block. Forces involved in making the connections to the terminal blocks using a screwdriver or the like, therefore can be transmitted through these terminal blocks to the I/O circuit card. Unless the terminal blocks are firmly mounted, movement of the terminal blocks under such forces can cause distortion of the attached circuit card and possible damage to its printed circuitry or components.
Precise, rigid and strong mountings of each of the circuit cards and the terminal blocks against the housing are extremely difficult to achieve. "Stack-up" of tolerances between the molded components of the housing and terminal blocks prevent designs requiring close fits between each of these parts and the circuit cards.